Slick Magneto 4370

what does Magneto Lag 20 mean?

It is the number of degrees that the ignition timing gets mechanically retarded while cranking the engine to start.

Lycoming specifies a specific lag (retard) angle for all of its engines Usually on the factory data plate I think). A large majority of them it is specified at 25 degrees but some it is 20.

Thanks Scott, i fell somewhat embarrassed, let me provide a little more background. I have the new IO-390-EXP installed, the timing calls for 20 deg BTDC. Per the data sheet that came with the engine, the Left Mag is a Slick 4372, and the Right is a 4370. From some other research I’ve done it looks like the Left 4372 has a 15 deg lag, and the Right has a 20 deg lag.
My engine starting practice was Left mag HOT and Right mag HOT "after" engine start, this procedure was based on something I read or assumed, but cant verify right now. I recently was experiencing cold (32F) starting issues, hard to start with lots of cranking, but when I started to turn on the Right Mag “during cranking”, the engine would start much easier. Another part of the problem was, I have never started the engine at that low of temperature, and I needed to learn a cold start recipe. My question now is, can I change my starting procedure with both Left and Right mags HOT at the same time without the risk of engine kickback, based on the mag lag numbers? Thanks for your help.

Not surprising that the more advanced timing helped during cold starts. Only extensive testing can determine if 5* BTDC is too much timing to avoid most kickback scenarios. It is not just the different lags. Having two mags firing at TDC is effectlively a much more advanced timing (i.e. faster staring) than just one mag firing at TDC. I have two EIs firing at TDC during startup and have never experienced a kickback. Can't say that would be the case if one of those was firing at 5 BTDC. I would at least run the mag with the 15* lag during startup in cold, instead of the 20.

Remember that fuel required for startup is not linear with temp decline, once below about 60*F. It is exponential. Many simply don't add enough excess fuel when temps get that cold.

Larry

OK i also received troubleshooting from Lycoming: The Slick 4372 would be an impulse coupled magneto with a 15 degree lag angle, the Slick 4370 would be plain non impulse coupled 0 degree lag magneto. In this configuration the left should be used for starting and both after the engine is running. If you are having trouble starting with left only this may be an indication of incorrectly set timing possibly due to timing drift, a faulty impulse coupling, fouled plugs, or other ignition related issue. I would perform a mag drop check to see if the left side is indicating excessive drops, verify that the timing is set correctly to 20 degrees BTDC, visually inspect the sparkplugs and test them or rotate them so they are firing off the other magneto to see if this makes a change, perform a high tension lead check, verify that the impulse coupling is snapping at the correct time by turning the prop over by hand and checking at how many degrees indicated the impulse coupling snaps at, should be within 5 degrees of 5 BTDC any more may indicate a faulty impulse coupling or excessive magneto drift.
looks like i have some more investigating to perform...
thanks everyone.